DESCRIPTION: TRF1 is a mammalian telomere binding protein identified and cloned by Dr. DeLange. The protein clearly resides at telomeres in stained cells. The structure of the protein reveals an acidic N-terminus, a dimerization domain, and a C-terminal myb DNA-binding domain. The mouse and human proteins are related along their lengths. In addition, there is a second telomere binding protein TRF2 which shows strong homology to TRF1 in the DNA-binding domain and weaker homology elsewhere. This protein has also been shown to bind to the telomere repeat sequence and to reside at telomeres in cells. A key observation leading to this proposal is that the length of telomeres in telomerase expressing cells can be influenced by varying the level of TRF1. Increased expression of TRF1 leads to shortening, while expression of a dominant negative construct (dimerization domain) leads to lengthening. These operations do not affect levels of telomerase by in vitro assay. The model for these findings is that the TRF1 at telomeres down regulates telomerase as a homeostatic mechanism to maintain telomere length. This is similar to a model in yeast involving RAP1. In this proposal the aims are: 1 Test a model for the role of TRF1 in telomere length control by tethering the protein using the GAL4 DBD. 2. Determine the contribution of TRF2 to telomere length control. 3. Create TRF1 and TRF2 null mutations in chicken cells. 4. Alter TRF1 levels in telomerase negative cells. 5. Inhibit TRF1 in human T-cells. 6. Knock out TRF1 in the mouse germ line.